forced out the discharge valve. Then it passes through
the fourth-stage intercooler and moisture separator.
Air is drawn into the fifth-stage cylinder on the
piston downstroke and is compressed and discharged on
the upstroke. The discharge air passes through the
fifth-stage pulsation bottle, the aftercooler, the moisture
separator, a back-pressure valve, and a check valve
before entering the ships HP piping.
Many hazards are associated with pressurized air,
particularly air under high pressure. Dangerous
explosions have occurred in high-pressure air systems
because of DIESEL EFFECT. If a portion of an
unpressurized system or component is suddenly and
rapidly pressurized with high-pressure air, a large
amount of heat is produced. If the heat is excessive, the
air may reach the ignition temperature of the impurities
present in the air and piping (oil, dust, and so forth).
When the ignition temperature is reached, a violent
explosion will occur as these impurities ignite. Ignition
temperatures may also result from other causes. Some
are rapid pressurization of a low-pressure dead-end
portion of the piping system, malfunctioning of
compressor aftercoolers, and leaky or dirty valves.
Air compressor accidents have also been caused by
improper maintenance procedures. These accidents can
happen when you disconnect parts under pressure,
replace parts with units designed for lower pressures,
and install stop valves or check valves in improper
locations. Improper operating procedures have resulted
in air compressor accidents with serious injury to
personnel and damage to equipment.
You must take every possible step to minimize the
hazards inherent in the process of compression and in
the use of compressed air. Strictly follow all safety
precautions outlined in the manufacturers technical
manuals and in the NSTM, chapter 551. Some of these
hazards and precautions are as follows:
1. Explosions can be caused by dust-laden air or by
oil vapor in the compressor or receiver if abnormally
high temperatures exist. Leaky or dirty valves,
excessive pressurization rates, or faulty cooling systems
may cause these high temperatures.
2. NEVER use distillate fuel or gasoline as a
degreaser to clean compressor intake filters, cylinders,
or air passages. These oils vaporize easily and will form
a highly explosive mixture with the air under
3. Secure a compressor immediately if you observe
that the temperature of the air being discharged from any
stage exceeds the maximum temperature specified.
4. NEVER leave the compressor station after
starting the compressor unless you are sure that the
control and unloading devices are operating properly.
5. Before working on a compressor, make sure the
compressor is secured. Make sure that it cannot start
automatically or accidentally. Completely blow down
the compressor, and then secure all valves (including the
control or unloading valves) between the compressor
and the receiver. Follow the appropriate tag-out
procedures for the compressor control valves and the
isolation valves. When the gauges are in place, leave the
pressure gauge cutout valves open at all times.
6. Before disconnecting any part of an air system,
be sure the part is not under pressure. Always leave the
pressure gauge cutout valves open to the sections to
which they are attached.
7. Avoid rapid operation of manual valves. The
heat of compression caused by a sudden flow of
high-pressure air into an empty line or vessel can cause
an explosion if oil or other impurities are present.
Slowly crack open the valves until flow is noted, and
keep the valves in this position until pressure on both
sides has equalized. Keep the rate of pressure rise
under 200 psi per second.
The removal of moisture from compressed air is an
important feature of compressed air systems. As you
have learned, some moisture is removed by the
intercoolers and aftercoolers. Air flasks and receivers
are provided with low-point drains so any collected
moisture may drain periodically. However, many
shipboard uses for compressed air require air with an
even smaller moisture content than is obtained through
these methods. Water vapor in air lines can create other
potentially hazardous problems. Water vapor can cause
control valves and controls to freeze. These conditions
can occur when air at very high pressure is throttled to
a low-pressure area at a high-flow rate. The venturi
effect of the throttled air produces very low
temperatures that cause any moisture in the air to freeze
into ice. Under these conditions, a valve (especially an
automatic valve) may become very difficult or
impossible to operate. Also, moisture in any air system
can cause serious water hammering (a banging sound)
within the system. For these reasons, air dehydrators or